Plant of advanced age captured by general staff (9)
I believe the answer is:
goldenrod
'plant' is the definition.
(goldenrod is a kind of plant)
'of advanced age captured by general staff' is the wordplay.
'of advanced age' becomes 'old' (advanced in age means old).
'captured by' means one lot of letters goes inside another.
'general' becomes 'gen'.
'staff' becomes 'rod' (both can mean a stick or cane).
'old' placed inside 'gen' is 'golden'.
'golden'+'rod'='GOLDENROD'
(Other definitions for goldenrod that I've seen before include "Yellow-headed garden plant" , "Plant with tall spikes of yellow flowers" , "Tall-spiked plant" , "Yellow-flowered plant" , "Autumnal plant" .)